A. Hodgson

A molecular perspective of water at metal interfaces

Water/solid interfaces are relevant to a broad range of physicochemical phenomena and technological processes such as corrosion, lubrication, heterogeneous catalysis and electrochemistry. Although many fields have contributed to rapid progress in the fundamental knowledge of water at interfaces, detailed molecular-level understanding of water/solid interfaces comes mainly from studies on flat metal substrates. These studies have recently shown that a remarkably rich variety of structures form at the interface between water and even seemingly simple flat surfaces. In this Review we discuss the most exciting work in this area, in particular the emerging physical insight and general concepts about how water binds to metal surfaces. We also provide a perspective on outstanding problems, challenges and open questions.

A one-dimensional ice structure built from pentagons

Heterogeneous ice nucleation has a key role in fields as diverse as atmospheric chemistry and biology. Ice nucleation on metal surfaces affords an opportunity to watch this process unfold at the molecular scale on a well-defined, planar interface. A common feature of structural models for such films is that they are built from hexagonal arrangements of molecules. Here we show, through a combination of scanning tunnelling microscopy, infrared spectroscopy and density-functional theory, that about 1-nm-wide ice chains that nucleate on Cu(110) are not built from hexagons, but instead are built from a face-sharing arrangement of water pentagons. The pentagon structure is favoured over others because it maximizes the water-metal bonding while maintaining a strong hydrogen-bonding network. It reveals an unanticipated structural adaptability of water-ice films, demonstrating that the presence of the substrate can be sufficient to favour non-hexagonal structural units.

Water at Interfaces

The interfaces of neat water and aqueous solutions play a prominent role in many technological processes and in the environment. Examples of aqueous interfaces are ultrathin water films that cover most hydrophilic surfaces under ambient relative humidities, the liquid/solid interface which drives many electrochemical reactions, and the liquid/vapor interface, which governs the uptake and release of trace gases by the oceans and cloud droplets. In this article we review some of the recent experimental and theoretical advances in our knowledge of the properties of aqueous interfaces and discuss open questions and gaps in our understanding.

State resolved desorption measurements as a probe of surface reactions

Surface reactions which lead directly to gas phase products can be investigated by using state resolved techniques to measure the energy released into the newly formed molecules. This technique has been used extensively to explore oxidation of CO and the dynamics of H2 recombinative desorption at surfaces, but so far has been applied to few other reactions. Here we review the application of final state measurements and discuss the conditions under which dynamical information can be obtained for Langmuir–Hinshelwood type surface reactions. Combining resonance enhanced multiphoton ionisation with ion time of flight detection allows translational energy distributions to be measured for a wide range of products, with full quantum state resolution. The energy release reflects scattering from a thermally populated transition state, with the recombination dynamics determining how the product state distributions depart from a thermal distribution at the surface temperature. Using the principle of detailed balance the desorption dynamics can be related to the reverse process, dissociative chemisorption. Making the link between adsorption and desorption has two benefits. Firstly, it allows us to discuss quantitatively the influence of surface temperature on the product state distributions formed by surface reactions, allowing us to avoid naive models, which treat the transition state as having a unique, well-defined energy. Secondly, the desorption results can be used to obtain relative sticking probabilities with full quantum state and translational energy resolution, providing a way to determine how internal energy influences dissociation for both hydrogen and for heavy molecules, such as nitrogen. The conditions necessary to apply detailed balance successfully are discussed and the desorption distributions expected for different types of adsorption behaviour illustrated. The recombination/dissociation dynamics of hydrogen are summarised briefly and the energy partitioning into different coordinates described. Product state measurements for some ‘heavy’ molecule reactions, such as NH3 and NO reduction to form nitrogen and CO and H2 oxidation are reviewed and compared to the behaviour seen for hydrogen. The desorption dynamics and the shape of the potential energy surfaces for nitrogen recombination at different metal surfaces are discussed and we suggest some future lines of development.

Adsorption and desorption dynamics of H2 and D2 on Cu(111): The role of surface temperature and evidence for corrugation of the dissociation barrier

We report the effect of surface temperature on the state resolved translational energy distributions for H2 and D2 recombinatively desorbed from Cu(111). Sticking functions S(v,J,E) can be obtained by applying detailed balance arguments and follow the familiar error function form at high energy, consistent with previous permeation measurements [Rettner et al., J. Chem. Phys. 102, 4625 (1995)]. The widths of the sticking functions are identical for both isotopes and are independent of rotational state. S(E) broadens rapidly with increasing surface temperature, with a low energy component which is slightly larger than represented by an error function form. This is similar to the behavior seen on Ag(111) [Murphy et al., Phys. Rev. Lett. 78, 4458 (1997)] but on Cu(111) the low energy component remains a minor desorption channel. The broadening of S(E) can be explained in terms of a change in the distribution of barriers caused by local thermal displacement of the surface atoms, thermal activation of the surfa...

Quantum state-selected photodissociation dynamics in H2O and D2O

Two photon excitation, tunable near 248·5 nm, has been used to dissociate H2O/D2O via the [Ctilde] 1 B 1 and [Btilde] 1 A 1 states. Rotationally resolved OH/OD(A 2Σ+) photofragment excitation spectra are reported, following excitation to predissociated levels of [Ctilde] 1 B 1. Rotational resolution of the OH/OD(A 2Σ+ → X 2Π) fluorescence, generated from individual J′ K a K c levels of [Ctilde] 1 B 1, allows full quantum state selection in both the entry and exit channels. The OH/OD(A 2Σ+) fragment is formed rotationally hot as a result of the large change in bond angle in going from [Xtilde] 1 A 1 (or [Ctilde] 1 B 1) to the linear dissociative [Btilde] 1 A 1 surface. Product alignment measurements allow assignment of the two photon continuum absorption to [Btilde] 1 A 1: a-axis rotation in [Ctilde] 1 B 1 destroys product alignment from these levels. Electronic branching from ⪷B 1 A 1 to A 1 B 1 (and/or [Xtilde] 1 A 1) during the dissociation forms ground state OH/OD(X 2Π). Relative branching ratios are o...

Photodissociation of H2O2 at 248 nm: translational anisotropy and oh product state distributions

Abstract Ground-state OH(X 2 Π) fragments, from the photodissociation of H 2 O 2 at 248 nm, have been probed by laser-induced fluorescence. Nascent product rotational distributions and polarisation-dependent Doppler lineshapes are reported. The high translational anisotropy (β = −1 at low N″ ) indicates a prompt repulsion along the O-O axis. Polarisation-dependent Doppler lineshapes are interpreted as a correlation between the fragment recoil velocity v and angular momentum J OH , the product rotation being aligned along v . Product rotation is generated by an axial torsion about the O-O axis, consistent with a change in dihedral angle in the upper state.

Inverted vibrational distributions from N2 recombination at Ru(001): Evidence for a metastable molecular chemisorption well

We have measured translational and internal state distributions for N2 desorbed from a Ru(001) surface following NH3 cracking at 900 K. Nitrogen is formed with a vibrational population inversion, P(v=1)/P(v=0)=1.4, but a subthermal rotational energy release, Trot(v=0)=630 K. The translational energy distributions show a peak at low energy with a tail extending up to ∼2 eV and a mean energy release of 0.62 eV for N2(v=0) and 0.61 eV for (v=1). The product state distributions indicate a preferential energy release into the N2 stretching coordinate with a relatively weak N2–surface repulsion. Density functional calculations for N2 dissociation on Ru(001) and Cu(111) have been performed to compare the shape of the potentials in the N2 stretching (d) and translational (Z) coordinates. These reveal a sharp curvature of the surface for Ru, the energy release occurring close to the surface over a narrow range of Z. We suggest that this behavior is the result of the presence of a metastable molecular state, bound ...

Deuterium dissociation on ordered Sn/Pt(111) surface alloys

We have explored the effect of alloying an unreactive metal, Sn, on the dynamics of D2 dissociative chemisorption at Pt(111). By comparing D2 sticking and recombinative desorption on Pt(111) with that on the ordered p(2×2) Sn/Pt(111) and (∛×∛)R30° Sn/Pt(111) surface alloys, we examine the influence of the local surface composition on reactivity. The energy dependence of D2 sticking S(E) has been measured for all three surfaces using a hyperthermal beam. We find that the activation barrier for dissociative chemisorption is low on the p(2×2) alloy, but the sticking probability is reduced, compared to Pt(111), by an increase in the steric constraint on the dissociation site. Sticking on the (∛×∛)R30° alloy is inefficient at thermal energies with a threshold of ∼280 meV, below which the sticking probability falls exponentially. The increase in the barrier to D2 dissociation occurs as the stable, high coordination Pt3–D binding sites are lost by formation of the (∛×∛)R30° alloy. Despite the large activation ba...

Dissociative chemisorption of O2 on Cu(110)

Abstract The initial sticking coefficient S0 for the dissociative chemisorption of O2 on Cu(110) has been measured as a function of translational energy, surface temperature and scattering azimuth using a molecular beam. Sticking is facile on this face, having an initial sticking coefficient S0 = 0.2 at 0.05 eV and rising to S0 = 0.8 at translational energies above 0.35 eV. The dissociation probability is independent of scattering azimuth and follows a normal (cos2θi) energy scaling. At low beam energies the sticking coefficient shows a strong surface temperature dependence, S0 rising to ∽ 0.6 at 100 K. For beam energies above ∽ 0.2 eV the sticking coefficient is independent of surface temperature. This is interpreted in terms of two competing mechanisms, a trapping-desorption channel which dominates at low translational energies and low surface temperature (Ts) and a direct, activated dissociative chemisorption channel which becomes important as the energy is increased above 50 meV. The trapping state is thought to be a weakly bound, physisorbed molecular state, while the weak temperature dependence at high beam energies indicates direct, activated dissociation with negligible kinetic competition due to desorption via an (unstable) molecularly chemisorbed state. The coverage dependence S(θ) changes from a linear dependence at high beam energy and surface temperature to a slower decrease with θ at low Ts, characteristic of an extrinsic precursor state. The trapping channel can be modelled as a function of θ and Ts using a simple kinetic model for the precursors, based on the kinetic parameters obtained for the intrinsic precursor state and assuming local ordering of the adsorbate, even at the lowest surface temperatures (100 K).